It is well known that the propagation velocity of ultrasonic waves through a sample gas is presented by a function of the concentration and the temperature of the sample gas. The velocity of ultrasonic waves C (m/sec) propagating through a sample gas is presented by following equation (1) with mean molecular weight M and the temperature T (K).C=(κRT/M)1/2  (1)Where;    κ: ratio of molecular specific heat at constant volume and molecular specific heat at constant pressure    R: gas constant
Therefore measuring the velocity of ultrasonic waves C (m/sec) propagating through a sample gas and the temperature T (K) of the sample gas will provide the mean molecular weight M of the sample gas through a calculation. For example, the mean molecular weight M of a sample gas containing an oxygen-nitrogen gas mixture of a mixture ratio P:(1−P)(0≦P≦1) will be calculated by M=MO2P+MN2(1−P), where MO2: Molecular Weight of oxygen and MN2: Molecular Weight of nitrogen. Therefore, the oxygen concentration P will be obtained through a calculation on the basis of the measurement of mean molecular weight M. When the sample gas is an oxygen-nitrogen mixture, κ=1.4 is reasonable over a wide range of the oxygen-nitrogen mixture ratio.
When the velocity of ultrasonic waves propagating through a sample gas is C (m/sec), and the flow velocity of the sample gas is V (m/sec), the velocity of ultrasonic waves V1 (m/sec) propagating in the forward direction relative to the sample gas flow is V1=C+V, and the velocity of ultrasonic waves V2 (m/sec) propagating in the backward direction relative to the sample gas flow is V2=C−V. Therefore, the velocity of the sample gas flow V (m/sec) is calculated by following equation (2).V=(V1−V2)/2  (2)
The flow rate (m3/sec) of the sample gas will be obtained by multiplying this by the sectional area (m2) of the conduit through which the sample gas flows.
Methods and apparatuses for measuring the concentration of a certain gas or the flow velocity of a sample gas, using the above principle, on the basis of the propagation velocity or the propagation time of ultrasonic waves through the sample gas have been developed. For example, Japanese Unexamined Patent Publication (Kokai) No. 6-213877 describes an apparatus for measuring the concentration and the flow rate of a sample gas by measuring the propagation time of ultrasonic waves propagating between two ultrasonic transducers opposingly disposed in a conduit through which the sample gas flows. Further, Japanese Unexamined Patent Publications (Kokai) No. 7-209265 and No. 8-233718 30 describe an apparatus for measuring the concentration of a certain gas contained in a sample gas by measuring the propagation velocity or propagation time of ultrasonic waves propagating through a control volume with a reflecting type apparatus including an ultrasonic transducer and an opposingly disposed reflector. Further, U.S. Pat. No. 5,060,506 describes an apparatus for measuring the concentration of a two-component sample gas by measuring the changes in the velocity of ultrasonic waves.
Such a method and an apparatus for measuring the concentration and the flow rate by using the propagation velocity of the ultrasonic waves have problems. In the above-described method and apparatus, the sample gas includes only two components of oxygen gas and nitrogen gas. However, an oxygen concentrating apparatus actually outputs an oxygen enhanced gas including argon gas in addition to oxygen gas and nitrogen gas. Further, the concentration of argon gas is not constant and changes depending on the flow rate of the oxygen enhanced gas generated by the oxygen concentrating apparatus. Therefore, the conventional ultrasonic concentration measuring device cannot measure the concentration of oxygen gas accurately.